How old should a child be before they are introduced to social networks?

Nowadays children are on social networks as young as ten! I remember when I had to lie about my age to be on sites such as Blackplanet and Facebook because you has to be 18 but, that was years ago. If you have a child, what’s the age you would allow your child to use social networks? If you don’t have a child when do you think a child should be able to use social sites?